How much thrust reduction can I expect from 4000ft altitude gain?

Hi all,
I am designing a drone to fly at an altitude of 7,000ft (2133m). The motors I am buying from KDE produce 1.98lbs of thrust each at 3,000ft (914m) with my chosen propellers. How much thrust reduction can I expect from this number at 7,000ft? Does a calculator exist for this calculation? I tried using eCalc but I cannot put in the correct motor and battery unless I buy the full version.
Thanks a lot in advance!

There is a good white paper here
Check out around page 44

This is a much shorter paper and presents the nitty gritty of the maths

thanks a lot for the links! I will check those out.

I plugged some guesses into ecalc and you can expect some slight loss of thrust with altitude, but it’s impossible to say more without knowing the specifics about your set up.
You might find a simple prop change is enough to compensate.

ecalc is not expensive and it’s worth every cent, even from a hobby perspective. For commercial operations it’d be essential. The cost is less than the difference between the price of one prop over another.

Ok. I will look into buying ecalc then. Not sure if my situation will allow me to purchase it. These are the propellers that I’m using.

For a higher altitude propeller, I would want a more positive pitch angle, correct?

I’ve pinged one of the KDE engineers (KDE are an ArduPilot Partner) in the hope they’ll have more accurate data than the eCalc approximation.

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thank you! I actually just purchased ecalc and I’m about to plug stuff in. I’ll report back here for anyone who’s curious.

So, looks like each of my motors will lose about .5lbs of thrust totalling to about 4lbs lost. However, even at altitude and with my expected payload, I am still at a 1.5 to 1 thrust to weight ratio which I think is not bad for my first drone ever and I think will work fine for my application.

Whoops, that was at 7,000 METERS not 7,000 feet. At 7000ft it will experience very small losses! Now I am back up to a 1.9 to 1 thrust ratio. This calculator is very useful! It also claims that I could use smaller motors with dual props instead of bigger motors with triple props and get the same performance.

I think that thrust goes directly with air density. At 7000 ft air density is 20% lower than on sea level, so you shoud expect 20% thrust reduction or a bit higher due to propeller not working at design point. I think this matches your calculation. At this high propeller design is not crucial yet, but one as you shiwed, wider at the root and thinner at the tip should perform well or better at this height than a more constant wide blade. If possible I would try to increase thrust to weight ratio. 1.5 could be close to thw limit, even more if something doesn’t behave as expected at this height.

1.9-1.5:1 is very low. in this heights strong winds are not uncommon, with your low thrust/weight you might get into trouble. it would go at least for 2.3:1 better 3:1 to be save in heavy winds.

I built a quad for just these circumstances.
A request to do line pulls in the highlands of New Guinea at around 5000m
At ground level it was ridiculously overpowered.
Here is a short video for your edification

From one of the KDE engineers, a little math :slight_smile:

U.S Standard Atmosphere Air Properties lists an air density of 1.112 kg/m^3 at 1000m and 1.007 kg/m^3 at 2000m.
1.007 / 1.112 gives us approximately 90% of the thrust output at 2000m.
With 1.98 lb of thrust at 1000m, you could expect about 1.78 lb at 2000m.

Air humidity and air temperature also greatly effect air density and the thrust output of the motor so, the calculation above is just an estimate.

Thanks! I appreciate you following through with this information

I am flying the drone indoors so wind is not an issue. The building is located at about 7,000ft ASL. I did manage to adjust some things and bring my T/W ratio up to 2.1. Thanks everybody for your help!